1. Field of the Invention
The present invention relates to a noncontact rotating angle detecting sensor that can detect a relative rotating angle between two members by using a permanent magnet and a magnetic sensor.
2. Description of Related Art
As a rotating angle detecting sensor of this type, a sensor including a columnar permanent magnet and a plurality of magnetic sensors that are arranged on the same circumference at a position axially spaced apart from one end face of the permanent magnet. The rotating angle detecting sensor of this type includes a permanent magnet fixed to one member of two members between which a relative rotating angle is desired to be detected, a plurality of magnetic sensors fixed to the other member, and a processing circuit that converts a rotating angle of a relative concentric rotation of the two members into a rotating magnetic field of the permanent magnet to calculate the rotating angle by an operation using an output difference between the plurality of magnetic sensors. In the fixing, as far as possible, a center line of the permanent magnet, center of the circumference on which the plurality of magnetic sensors are arranged, and rotating centers of the relative rotation between the two members are arranged on the same straight line (Unexamined Japanese Patent Publication No. 2004-191101).
Conventionally, as the permanent magnet, a permanent magnet that is obtained by radially double-pole-magnetizing a columnar shaped body is used. The permanent magnet of this type has the N pole on a half and the S pole on the other half, when the magnetic field is divided by an axial plane including the center line of the columnar shape. The plurality of magnetic sensors detects magnetic flux densities to detect a rotating angle. As far as possible, the plurality of magnetic sensors are advantageously arranged in a space having a large absolute value of a magnetic flux density to avoid a gap between the magnetic sensor and one end face of the permanent magnet from being narrowed. For example, when an axial magnetic flux density along the center line of the permanent magnet is detected by the plurality of magnetic sensors, since a peak of an axial magnetic flux density distribution curve is located at a position axially spaced apart from an outer edge of one end face of the permanent magnet, the position is set as a normal arrangement position of the plurality of magnetic sensors (Unexamined Japanese Patent Publication No. 2005-291942).
However, when the axial magnetic flux density distribution in a space axially spaced from one end face of the permanent magnet is considered on an arbitrary plane including the center line of the permanent magnet, a curve representing the magnetic flux density distribution has a narrow radial region in which the curve has a moderate inclination near the peak. For this reason, when so-called decentering in which the center of the columnar permanent magnet and the center of the same circumference on which the plurality of magnetic sensors are arranged are relatively decentered occurs, each of the magnetic sensors enters a diametric region having a sharp magnetic flux density change, an output different from an output from a magnetic sensor corresponding to an actual rotating angle is obtained, and an angle detection error between a rotating angle calculated by a rotating angle detecting sensor and the actual rotating angle between the two members easily becomes large. In order to reduce the angle detection error, the rotating angle detecting sensor disclosed in Unexamined Japanese Patent Publication No. 2005-291942 employs a permanent magnet having a crucible former or a stepped shape at one end in place of the columnar permanent magnet. However, the permanent magnet is difficult to be designed by the special shaping.